The photo−ammonification process plays a crucial role in the transformation of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN). However, previous studies have primarily focused on DON biotransformation than on abiotic processes. This study investigated the photo−ammonification process of nine model low molecular weight (LMW) DON molecules (e.g., amino acids, nucleotides, and urea) under the influence of different light sources. The results showed that photo−ammonification of model DON was mainly induced by UV light, while negligible contribution by visible light was found. Depending on their molecular structures, amino acids yielded different ammonia amounts, whereas negligible photo−ammonification was observed for nucleotides and urea. As for the reactive species, ·OH promoted ammonia yields of all the model amino acids; ³CDOM^⁎ contributed to the photo−ammonification of six amino acids; ¹O₂ only had a positive impact on ammonification of tryptophan, histidine, and tyrosine; and CO₃·⁻ accelerated ammonia generation from histidine and methionine. In natural water samples, tryptophan, tyrosine, histidine, and methionine generated significant ammonia. ·OH and ¹O₂ were speculated as the contributing reactive species based on kinetic studies as well as significant fluorescent humic−like and tyrosine−like substances degradation in irradiated samples compared to the raw samples characterized by the EEM−PARAFAC analysis. The negative linear correlations between photo−ammonification rates and the E_LUMO−E_HOMO of the amino acids emphasized the importance of the role of the molecular structure. Overall, these results revealed the LMW DON photo−ammonification mechanism in sunlit surface waters and highlighted its significance in the nitrogen biogeochemical cycle as well as water quality management.